Unlike most portable Mario sports titles, there's no roleplaying mode in Mario Tennis Open. Nintendo Power asked Hiroyuki Takahashi, one half of Camelot's boss brothers, about the omission.
Takahashi explained that this time around, Camelot was trying to create a console-quality experience with features like online play and download matches, but one of the big reasons behind omitting the RPG mode was the inclusion of Mii characters:
Another reason is that the game's cast includes Mii characters in addition to characters from the Mario family, which are difficult to work into an [RPG] scenario. It didn't feel right to have already-existent Mii characters appear in an original storyline. But there are RPG-like elements in the form of costumes. I hope people will enjoy collecting the various costumes to customise their own characters and expand their abilities.
Having created the Shining Force and Golden Sun series, we're sure Camelot could have made Miis work in a Mario story, but we won't get to see that this time.
We recently played the game at PAX East and think it's a bit good. Here's our First Impressions: Mario Tennis Open to tide you over until the game's release in late May.